What are the benefits of modeling in one piece, or as separate parts?
The way I would go about that example, there would be separate objects for the hub and the blade. In the situation you described there is no reason to add vertices to the hub and extrude the blades out. In a real propeller there are separate pieces. There is no need to make modeling and texturing the propeller harder then it needs to be.
Another way to look at it, what do you gain by having the blades extruded from the hub? From modeling, texturing, rigging and animating in this case it is all easier with separate objects.